Image display devices are used in various fields such as TVs, mobile phones, monitors for personal computers, digital signages, and other industrial monitors. In recent years, usages of such image display devices have been expanded, and opportunities of using image display devices for, for example, display devices in video-conference systems, and further, using image display devices on the medical front have been increased. Among such image display devices, required performances are largely different from generally used image display devices such as TVs and monitors for personal computers. In the future, for the image display devices to accommodate to and make inroads in broader fields, it is necessary to satisfy display characteristics corresponding to an intended use of a corresponding field and also a technology development aiming at that satisfaction is necessary.
Typical image display devices for medical use include an electronic medical chart monitor, an X-ray image monitor, a monitor for surgical operation, a monitor for remote medical care, etc. A specific example will be explained by, for example, the monitor for remote medical care. As the name represents, the monitor for remote medical care is an image display device used in what they call “remote medical care” for doctors to diagnose patients from a distance. Particularly, as a shortage of doctors is being pointed out in this day and age, a system for seeing doctors regardless of places is mandatory. In such a remote medical care, based on a picture of a patient shown on a monitor, a doctor makes various diagnoses. To grasp the status of the patient, it is necessary to grasp the color of the patient's face, a status of the patient's skin, and a status of an inflamed area in detail, and diagnoses of symptoms etc. are made based on observation results of these factors. In this situation, the picture itself shown on the monitor is one piece of data for the doctor to know the status of the patient. Therefore, the color, contour and texture of human skin are necessary to be expressed on the monitor. Such a request in characteristics is a special request and different from those image display devices such as TVs.
Also, video-conference systems which have been spread in recent years are systems which bi-directionally display images of both sides in conference rooms at different places on image display devices via the Internet or a telephone line. The video-conference system artificially creates an atmosphere such that both are debating in the same conference room. As to such an image display device, it is desired to express the color and texture of skin and presence etc. of the other persons in detail.
Note that bi-directional visual systems such as medical systems like the monitor for remote medical care and a monitor for surgical operation etc., and the video-conference system are getting easier to install than before as the Internet has been popularized. There is a trend of further broadening the uses in the future.
Image quality of an image displayed on an image display device such as colors and brightness etc. are determined by a gamma correction. Gamma correction means a correction operation by adjusting a relative relation of a picture signal (input value) from an external device and a signal (output value) upon display on the image display device so that people can watch the image without a feeling of strangeness. Generally, the input value is a gray level “x” in the picture signal, and the output value is expressed by a luminance relative value Y(x) when a luminance at a maximum gray level xmax is 1. A curve illustrating a relationship between the input value and the output value is called a “gamma curve.”
Currently, an 8-bit signal is used as a general picture signal, and the gray level “x” is expressed by an integer ranging from 0 to 255.For example, when a signal is an n-bit signal, the gray level “x” is an integer value ranging from 0 to 2n−1. And, a luminance relative value Y(x) is accordingly determined in a one-to-one relationship. When one gray level “x” is determined, one luminance relative value Y(x) corresponding to the gray level “x” is determined. While the gamma curve is a curve illustrating a relationship of the gray level and the luminance relative value, the gamma curve is not actually a continuous line on the coordinate but is a set of discrete points.
Conventionally, the gamma curve has been approximately expressed by a curve of Y=(x/(2n−1))2.2. This is because it has been empirically known that, by using the relationship, humans feel that the image is in a high image quality when they view TV pictures etc.